In traditional printing systems, where toner is used to provide marks on paper, the toner is typically housed within a cartridge in the system. The toner may also be transferred to a sump for use in the printing process. Since not all of the toner is used during each printing cycle, amounts of the toner may be resident within the cartridge or sump for extended periods of time. During this time, the unused toner may become electromechanically fatigued.
Accordingly, many types of print engines require a periodic purge of toner from the housing to maintain image quality. A purge routine is a non-productive printing mode, from a document processing standpoint, in which a high area coverage equivalent of toner is developed and sent to a cleaner. No printing occurs during this non-productive mode, or dead cycle, so as not to interfere with print jobs.
More particularly, during the purge cycle, the sump in the system is emptied of the toner by developing the toner onto a photoreceptor. The toner on the photoreceptor is then cleaned off the photoreceptor or printed onto paper. The sump is then refilled with fresh toner. The functions of emptying and refilling could be done concurrently or sequentially in traditional purging systems. This maintenance routine, though, results in an overall productivity reduction. Although net productivity (when image quality is considered) may be increased, overall productivity (also taking into account raw throughput) is still typically not achieved to desired levels.
Further, purge routines are typically triggered when some metric exceeds a threshold and is halted when either the same or different metric crosses a halting threshold (it may be as simple as an elapsed period of time). Example trigger metrics used in the past have been percent toner concentration (% tc) (which is a sensed value of the mass of toner particles over the mass of carrier particles) and toner age (an estimate of the average residient time population of toner particles in the sump). The purge routine will manage the metrics within some operable range.
In at least some situations, the goal is to maintain the average time of residence for toner within the sump to less than some upper threshold, for example, eighty (80) minutes. Algorithms have been implemented to calculate average residence time of toner within a sump or cartridge. However, when the purge process is used to maintain this average below an upper threshold value, there is a resulting drop in production. For example, a system that normally processes 100 prints per minute may only achieve a productivity level of 95 prints per minute when document area coverages are such that purging is triggered frequently. As such, the customers may not be satisfied. It is, therefore, desirable to reduce the productivity cost of the dead cycle while maintaining adequate image quality.
To improve productivity, a purge-while-run routine has also been proposed. This method continually develops additional images outside of the customer zone. Though possibly effective, it is expected that purge-while-run will strain cleaner latitude and, that for most pitch modes, the additional area to develop toner outside the customer zone is severely limited.